RKKY (Ruderman-Kittel-Kasuya-Yosida) coupling is a particular type of magnetic coupling due to interaction between the conduction electrons, making possible the creation, and widespread use of, synthetic AntiParallel (SyAP) structures (AP2/Ru/AP1). For MgO barrier based tunneling magneto-resistive (TMR) sensors, a typical structure would comprise:
Seed/antiferromagnetic (AFM) layer/AP2/Ru/AP1/MgOx/free layer/capping layer.
A ruthenium thickness of 7.5 Angstroms (Ru7.5) is commonly used in the AP2/Ru/AP1 portion because it is easy to control and is not too sensitive to Ru interfacial condition changes. However, as device and film stack size become ever smaller, the pinning field, which is limited by the saturation field of the SyAP (Hs), has to be correspondingly larger to ensure device stability.
Although the saturation magnetization Hs typically increases with the reduced magnetic moments of the AP2 and AP1 layers, and also with the moment difference between AP2 and AP1, the MR ratio will suffer once the layers become thinner than some critical value. Thus, at some point, SyAP with Ru7.5 will no longer be capable of producing sufficient Hs.
In current MgO based TMR, a higher annealing temperature will typically produce a higher TMR ratio and hence higher amplitude. So it would be desirable to anneal the TMR sensor at a higher annealing temperature if there were no adverse effects on other magnetic properties such as free layer coercivity and pinning strength. However, a comparison of FIGS. 1(a) and 1(aa), for a Ru7.5 SyAP shows how going to a higher annealing temperature, such as from 280° C. to 320° C. (for 2 hours in each case), reduces Hs from 7,600 to 6,000 Oe. This reduction in pinning field strength is even greater for the Ru4 SyAP system as can be seen by comparing FIGS. 1(b) and 1 (bb) where the synthetic exchange coupling has been almost destroyed. Note that in FIGS. 1(a) through 1(bb) relative units are used on the Y-axis. Hs is defined as the minimum value of the applied field (X-axis) beyond which the induced magnetization (Y-axis) remains unchanged within 5%.
Thus, further optimization of the SyAP is necessary to ensure a robust pinning field despite the use of higher temperature annealing such as 300° C. and above. However, to improve the magneto-resistive (MR) ratio of a tunneling MR (TMR) sensor, a higher annealing temperature is needed. So the problem outlined in the preceding paragraph has to be dealt with if we are to be able to achieve higher amplitudes for TMR sensors.
A routine search of the prior art was performed with the following references of interest being found:
U.S. Pat. No. 7,780,820 (Zhao et al—Headway) teaches that the annealing temperature needs to be lower than 300° C. U.S. Patent Application 2002/0097537 (Shimizawa) teaches first annealing at a temperature of 250° C., then at a lower temperature of 210° C. U.S. Patent Application 2009/0027810 (Horng et al—Headway) shows a CoFe layer on Ru under the AP1 layer. U.S. Pat. No. 7,377,025 (Zhao et al—Headway) discloses a plasma treatment of the AP1 layer, but not prior to forming the Ru layer.